A Beginner’s Guide to Object Oriented Programming (2)

Long time no see. I hope you still remember my recent posting on object-oriented programming. Here is part two, which I hope you are interested in. Today, we are going to have a look at inheritance. If you do not feel familiar enough with classes and objects, please refer to my posting “A Beginner’s Guide to Object Oriented Programming (1)”.

What’s inheritance got to do with programming?

In real life scenarios, an object of a specific class may be able to play different roles. Here is an example:

I am a person, and I have all the attributes and methods that are inherent to any person on this planet. I have certain height and weight. I can sleep(), eat(), and speak(). However, I always play a particular role which depends on the situation I’m in. I’m a teacher, husband, paramedic, scuba diver, runner, motorist, air passenger, …

Now, here comes an awesome fact: Each role that I play can be considered a specialized person. I play many roles, but yet I’m still only one single person. (Luckily … because if this wasn’t the case I’d be suffering from some serious mental disorder.) 😉

There is a term computer science which describes this situation: polymorphism (from Greek polymorphia, which means multiple forms).

Note, that whatever role I’m currently playing I still have all the methods and attributes of a person. For example, my teacher alter ego also has a certain height, weight, and all the capabilities (methods) that any other person may have. But what characteristics and abilities are specific to a teacher? You’ll probably agree that a teacher is able to teach, and that a teacher has to teach a certain number of classes per week.

In order to understand, how the situation above is being modelled in object-oriented programming we need to remember this: A class is a blueprint that can be used to build (instantiate) objects. What we are going to do now is to take this blueprint, put it on a xerox machine, and manually scribble in some features to get a new (extended) blueprint. The new blueprint now allows us to build objects that have both the inherited features and the new features. Of course, we can again make a copy of the new blueprint and add even more features.

The following UML class diagram shows a Teacher class that inherits attributes and methods from a class Person.


In computer science terminology, we say that class Teacher has been derived from class Person. Teacher is the superclass of Person, and Person is a subclass of Teacher. Be aware that I just wrote “the superclass” and “a subclass”. Any class should have only one direct superclass, but it may have more than one subclasses. Java and C#, among other programming languages, enforce single inheritance. (C++ does allow multiple inheritance, but if you feel like you need to use it in your code, this is usually the sign of bad OOP design.)

Person may have a superclass, such as Creature. Likewise, we can derive further classes from Teacher. Consider a school’s principal, for example. He is still a teacher, but he has additional skills, abilities, methods, whatever you want to call it. (Yay! At least in OOP class hierarchy, the principal is down below his school’s teachers.) 😉

Benefits of inheritance

One of the benefits of inheritance is pretty obvious: Reusability, which means that you don’t have to write the same (erroneous) code multiple times. Given the fact that a person, a principal, a teacher, and a plumber eat in exactly the same way, it is enough to implement this method only once.

The second benefit I’d like to mention is very similar to what’s going on in the real world. We know how to eat() and we know about the consequences of eating (some people hate ’em). 😉 However, most of us don’t have a clue about what is going on inside the eat() method. We don’t know the “guts”, so to speak. In our daily life, we appreciate objects that we can just use, without worrying about what’s going on inside. This is called Information hiding.

The third major benefit is Extensibility. As I have mentioned above you can easily enhance your model by adding more subclasses, without impacting the functionality of any of the other existing classes.

There are further benefits that you may discover as you gain experience in object oriented programming. Polymorphism, for example, allows you to write methods that accept objects of different types. The following code in Java shows the definition of method which can be invoked by passing a person, a teacher, a principal, or anything else within the class hierarchy below Person.

public static void giveThisPersonSomethingToEat( Person p ) {

I hope this tiny and incomplete lesson has helped you to understand the very basics of inheritance. If you have further questions, feel free to ask me in my lectures.

— Andre M. Maier

About bitjunkie

Teacher, Lecturer, and BITJUNKIE ...
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